A bathroom weighing scale is a commonly used piece of apparatus that allows a user to weigh themself to track their weight. Typically a user will weigh themself at the same time each day, for example when they wake up and enter the bathroom in the morning. Advanced bathroom scales are available that connect, via WiFi, to the internet and allow a user's weight to be remotely logged. The user can then review how their weight has varied over a period of time. However if the user forgets to weigh themself then data is missed and the history of their weight is incomplete.
It is known that a weight sensing unit can be placed under the leg of a bed allowing a component of the weight of the bed plus the weight of the user to be measured. By placing a weight sensing unit under each foot, the total weight of the bed and user can be measured as well as gross changes in weight due to incidents of the user leaving and then returning to bed. Analysis of the variation in the recorded weight will allow determination of the weight of the user by subtracting the weight recorded after the user has left the bed (signified by a large drop in total weight) from the total weight before such an event. In such a manner the user's weight can be reliably logged every day without the user needing to actively switch on and stand on a bathroom weighing scale. An example of such a system can be seen in EP 2148179 A1.
However, a simple weight measuring process using weight sensors positioned under the legs of a bed is only able to give a single weight reading for the whole bed. If two people are using a double bed then changes in weight can not be distinguished between the two users. Such a system therefore has limitations for multiple bed users.
A weight sensing unit that is to be placed under each foot of a bed will typically consist of a load cell element upon which are mounted one or more strain gauges, a base and a platform, as well as any necessary wires and cables for the strain gauge(s) to be wired e.g. in conventional Wheatstone bridge arrangements. There are a number of typical low profile weight sensors or load cells in the prior art, although not especially designed for use as bed scales. One type is described in U.S. Pat. No. 5,392,848, in which a cantilever load cell is positioned between a base and a platform. However the loads encountered beneath a centre leg of a double bed could rise to 150 kg or more and this type of load cell would need to be excessively thick to accommodate such high loads. A thinner alternative is described in U.S. Pat. No. 5,510,581 and such a design is capable of accommodating high loads without excessive thickness. However such a load cell requires fixing to a platform plate for use as well as requiring a central foot to deliver the central load resulting from its contact with the ground.
U.S. published application 2008/0041638 shows a load cell with a circular load cell comprising a thick circular base with a central contact knob that ensure that a central load is transmitted perpendicular to the load cell either directly or via a platform. Such a load cell may be used for indirect measurement of the weight of objects supported on a platform, such as a pallet or floor. A similar circular load cell is described in U.S. published application 2011/0067502 and again shows a thick circular base and with the option of a central ball bearing to apply the load perpendicular to the load cell via a platform. Finally U.S. published application 2010/0155152 shows a low profile load cell with a central raised bump that again ensures that the central load is transmitted perpendicular to the load cell via contact with a foot element. The base of this load cell does not have a thick section and is designed so that it can rest on a supporting surface without the need for additional fasteners. The same patent application shows an example of a further load cell in the prior art in FIG. 13, again with a central bump and without a thick base.
The various load cells known in the prior art have not been adapted for use in weight sensors intended to bear the weight of bed and be sensitive to changes in weight e.g. when a person moves in their sleep. Moreover it has not previously been possible to independently monitor the weight and weight shifting of two people sharing the same bed.